When a voltage higher than a power source voltage is required, a DC-DC converter using an inductor has been conventionally used as a power supply circuit. DC-DC converters have been used for many applications since a voltage at a desired level can be generated and power can be efficiently supplied to a load which consumes a large amount of current. However, it is difficult to downsize the DC-DC converter since components such as a transformer and a coil are required. Therefore, it has been impossible to include all components of the DC-DC converter in a semiconductor integrated circuit.
In view of these problems, a charge pump circuit has been sometimes used as a power supply circuit since the charge pump circuit can be downsized and is capable of high efficiencies. However, since the charge pump circuit raises an output voltage by adding a voltage of a capacitor charged by a voltage of a DC power source, the output voltage of the charge pump circuit is largely dependent on the power source voltage. Further, when a battery is used as the DC power source, the output voltage of the charge pump circuit decreases in accordance with a drop of a battery voltage, by a voltage level obtained by multiplying the drop in the battery voltage with a raising ratio of the voltage. As a result, the output voltage of the charge pump circuit rapidly decreases.
In order to avoid such effects of a change of the power source voltage, the following three methods have been suggested. As a first method, as shown in FIG. 5, a power source voltage VCC is controlled to be a constant voltage by a voltage regulator 101 and inputted to a charge pump circuit 102 to be raised. The raised voltage is then supplied to a load 110 (for example, see Patent Document 1). As a second method, as shown in FIG. 6, an input power source voltage VCC is inputted to the charge pump circuit 102 to be raised. The raised voltage is then inputted to the voltage regulator 101 and controlled as a constant voltage, and supplied to the load 110. As a third method, as shown in FIG. 7, when an output voltage Vout of a charge pump circuit becomes at a predetermined detection voltage or higher, each on-period of a transistor M101, which connects a flyback capacitor C1 in the charge pump circuit and a catch-back capacitor Cout provided outside the charge pump circuit, is forcibly shortened to obtain a constant output voltage.
[Patent Document 1] Japanese Patent Application Publication No. 2006-320158
An output voltage can be stabilized with stable ripple amplitude by the first and second methods. In these cases, however, there is required a voltage regulator which has a large output driver through which a maximum output current of the charge pump circuit can flow. Therefore, there is a problem in that chip size is increased.
By the third method, a constant voltage can be stably outputted by only monitoring an output voltage of the charge pump circuit and when the output voltage of the charge pump circuit becomes a predetermined level or higher, feeding back a detected signal to a clock unit which controls ON/OFF of the charge pump circuit. However, since each on-period of the charge pump circuit is forcibly shortened, there is a problem in that a ripple amplitude of the output voltage becomes larger.